Rotary cutting tool

ABSTRACT

A rotary cutting tool or end mill is provided, the tool comprising a plurality of pairs of diametrically-opposed, symmetrical, helical flutes formed in a cutting portion of the tool body, wherein the pitch between at least one pair of adjacent helical flutes is less than or greater than the pitch of at least one other pair of adjacent helical flutes in at least one radial plane along the axial length of the flutes, a plurality of peripheral cutting edges, wherein at least one of the peripheral cutting edges has a radial rake angle different from radial rake angle of a peripheral cutting edge of a different helical flute.

This application is a continuation of U.S. patent application Ser. No.12/876,538 filed Sep. 7, 2010, which is a continuation-in-part of U.S.patent application Ser. No. 11/953,748 filed Dec. 10, 2007, which issuedon Sep. 7, 2010 as U.S. Pat. No. 7,789,597, which is a continuation ofU.S. patent application Ser. No. 11/420,874 filed May 30, 2006, whichissued on Dec. 11, 2007 as U.S. Pat. No. 7,306,408, which claimspriority of U.S. Provisional Application No. 60/766,241 filed Jan. 4,2006, each of which are hereby incorporated by reference.

BACKGROUND AND SUMMARY

The present invention relates to a rotary cutting tool, and moreparticularly to an end mill having a plurality of pairs ofdiametrically-opposed, symmetrical, helical flutes, wherein the pitchbetween at least one pair of adjacent helical flutes is less than orgreater than the pitch of at least one other pair of adjacent helicalflutes in at least one radial plane along the axial length of theflutes, a plurality of peripheral cutting edges associated with theplurality of the helical flutes wherein at least one of the peripheralcutting edges has a radial rake angle different from radial rake angleof a peripheral cutting edge of a different helical flute. The improvedend mill provides reduced chatter, improved surface finish, and otheradditional benefits.

Rotational end mills have long been utilized for various cutting duties.Conventionally, these end-mills are constructed with different types ofhardened steel as well as tungsten carbide, and are often givenadditional structural features such as a corner radius at the cuttingends, tapered cutting ends, ball shaped cutting ends, uneven cuttingedges for rough milling operations including serrations and still otheredge contours. Likewise, these end-mills can be provided for longer wearwith wear-resistant PVD and CVD coatings including amorphous diamond andvarious nitride compositions.

A common problem encountered in the use of end mills is “chatter”. Whencutting ferrous and non-ferrous materials, especially at aggressivecutting feed rates, harmonics can generate regenerative vibrationwhereby the rotating end mill's frequency of vibration self-excites. Theself-exciting chatter is usually accompanied by a loud and excessivenoise while machining. One cause of this chatter is when the peripheralcutting edges formed along the helix are spaced at equal distances aboutthe end mill such that the time between the cutting edges hitting thematerial being cut is the same (or even worse, in a concave radial cutwhen more than one cutting edge hits the material being cut at the sametime and at the same intervals). Excessive chatter can result in a poorsurface finish, rework or scrap of the work product. Chatter can damagethe cutting edge of an end mill and limit its useful life, therebyincreasing costs for the milling operation and generating less precisemachined parts than may be desired or required for a particular finalfunction. Excessive chatter can also cause premature wear to the actualmilling machine and its components.

In order to combat the harmonics, variable helix end mills and variablepitch end mills have been developed. A variable helix end mill isgenerally an end mill having helical flutes in which the circumferentialdistance between the peripheral cutting edges varies in an axialdirection along the end mill. The circumferential distance is alsosometimes described as an angle between adjacent peripheral cuttingedges known as the index angle or pitch. One type of variable helix endmill is when adjacent helical flutes have different helix angles.Another type is when the helical flutes have different variable helixangles (i.e. the helix angle of one flute is 40 degrees at the leadingend of the flute and 35 degrees on the trailing end of the flute). Theother type of end mill discussed is the variable pitch end mills. Onetype of variable pitch end mill is when all helical flutes have the samehelix angle with the flute indexing altered from the typical 90 degreespacing. Unlike the variable helix end mills, the circumferentialdistance between adjacent peripheral cutting edges of a variable pitchend mill typically is constant in the axial direction of the end mill.

One of the most commercially successful variable helix end mills is theZ-Carb® end mill manufactured under U.S. Pat. No. 4,963,059, and ownedby the Applicant. The U.S. Pat. No. 4,963,059 patent disclosed an endmill having a plurality of paired helical flutes forming an even numberof helical peripheral cutting edges equally spaced circumferentially inone plane wherein the peripheral cutting edges are formed as a pluralityof pairs of diametrically opposite cutting edges having the same helixangle and thereby being symmetrical with respect to the axis of thebody. While the Z-Carb® end mill is resistant to chatter and provides agood surface finish, the technology is over 20 years old and it isbelieved that there is still room for improvement.

Many manufacturers of end mills have attempted to employ differentstrategies for reducing harmonics. One such attempt is described in USPublished Patent Application US2004/0120777, which teaches an end millhaving a plurality of flutes wherein each feature of the flute isunsymmetrical with each other flute including location of the fluteabout the tool (index angle), helix angle, radial rake angle, and radialrelief angle. It would seem to follow that a tool having everythingdifferent would be the best performer in terms of the reduction ofchatter, however, testing of these tools have shown a decrease inperformance in comparison to other leading end mills. An end mill havingall different features can have stability problems that may be evenworse than the problems with chatter. Another problem with such tools isthat the production and resharpening of the tool is difficult becauseall of the features of the end mill are different.

Another attempt to reduce harmonics and increase performance is taughtin U.S. Pat. No. 6,997,651, entitled End Mill Having Different AxialRake Angles and Different Radial Rake Angles. This prior art end millhas a plurality of flutes all having the same helix angle and beingequally spaced about the circumference of the tool (same index angle),but having at least two different radial rake angles and at least twodifferent axial rake angles. Like the other prior art end mill discussabove, the testing of this tools has shown a decrease in performance incomparison to other leading end mills, including in the reduction ofharmonics. The performance of this end mill will be discussed in greaterdetail below.

Many other attempts have been made in the prior art to improve theperformance of end mills with regards to chatter. The reduction ofharmonics is not accomplished by making all features different in arandom manner as this may have serious adverse consequences with theperformance of the tool. Accordingly, there remains room for improvementin the prior art to reduce chatter without sacrificing stability of thetool.

The present invention overcomes at least one disadvantage of the priorart by providing A rotary cutting tool comprising: a body having acutting portion and a shank portion; a plurality of pairs ofdiametrically-opposed, symmetrical, helical flutes formed in the cuttingportion of the body, wherein the pitch between at least one pair ofadjacent helical flutes is less than or greater than the pitch of atleast one other pair of adjacent helical flutes in at least one radialplane along the axial length of the flutes; a plurality of peripheralcutting edges associated with the plurality of the helical flutes;wherein at least one of the peripheral cutting edges has a radial rakeangle different from radial rake angle of a peripheral cutting edge of adifferent helical flute.

Still another embodiment of the invention overcomes at least onedisadvantage of the prior art by providing a rotary cutting toolcomprising: a body having a cutting portion and a shank portion; aplurality of pairs of diametrically-opposed, symmetrical, helical flutesformed in the cutting portion of the body, at least one flute beingformed at a constant helix angle, the pitch between adjacent helicalflutes being variable along the axial length of the flutes, and thepitch between all of the helical flutes being equivalent in at least oneradial plane of the cutting portion of the body; a plurality ofperipheral cutting edges, the peripheral cutting edges formed along anintersection of a circumferential surface of the cutting portion of thebody and a portion of an inner surface of a respective one of thehelical flutes facing in a direction of rotation of the body; wherein atleast one of the peripheral cutting edges has a radial rake angledifferent from radial rake angle of a peripheral cutting edge of adifferent helical flute; wherein within each of the pairs ofdiametrically-opposed, symmetrical, helical flutes: the radial rakeangle of one of the peripheral cutting edges of a pair of flutes isequivalent to the radial rake angle of the other peripheral cutting edgeof said pair of flutes; wherein the radial rake angle of at least one ofthe peripheral cutting edges is constant along the length of the helicalflute forming the peripheral cutting edge; and wherein all of theperipheral cutting edges have a positive radial rake angle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a rotary cutting tool cutting toolin accordance with the present invention;

FIG. 2 is a cross-sectional view of the rotary cutting tool of FIG. 1;

FIG. 3 is a cutting end view of the rotary cutting tool of FIG. 1;

FIG. 4 is a cutting end view of the rotary cutting tool of FIG. 1 withadditional detail views of the peripheral cutting edges of an embodimentof the present invention;

FIG. 5 is a cutting end view of a rotary cutting tool having additionaldetail views of the peripheral cutting edges of an embodiment of thepresent invention;

FIG. 6 is a cutting end view of a rotary cutting tool having additionaldetail views of the peripheral cutting edges of an embodiment of thepresent invention;

FIG. 7 is a detail cross-sectional view of a peripheral cutting edge ofan embodiment of a rotary cutting tool of the present invention showinga K-land;

FIG. 8 is a side elevational view of a rotary cutting tool cutting toolin accordance with another embodiment of the present invention showingvariable rake angles along a single helix;

FIG. 9 is an end view of an embodiment of another embodiment of thepresent invention having all different radial rake angles;

FIG. 10 is a side elevational view of a variable pitch rotary cuttingtool cutting tool in accordance with another embodiment of the presentinvention having equal helix angles;

FIG. 11 is a side elevational view of a rotary cutting tool cutting toolin accordance with another embodiment of the present invention; and

FIG. 12 is a cross-sectional view of the rotary cutting tool of FIG. 11taken at the midpoint of the length of cut.

FIG. 13 is a graph of a sound measurement taken during a cut made usinga prior art Z-Carb® end mill;

FIG. 14 is a graph of a sound measurement taken during a cut made usinga rotary cutting tool in accordance with one embodiment of the presentinvention;

FIG. 15 is a graph of a sound measurement taken during a cut made usingan end mill made in accordance with U.S. Pat. No. 6,997,651;

FIG. 16 is a chart showing a comparison of the sound measurements of thetools of FIGS. 13-15;

FIG. 17 is a chart showing a comparison of the surface measurements ofthe tools of FIGS. 13-15;

FIG. 18 is a picture showing an actual cut made using a prior artZ-Carb® end mill;

FIG. 19 is a picture showing an actual cut made using a rotary cuttingtool in accordance with one embodiment of the present invention;

FIG. 20 is a picture showing an actual cut made using a prior art endmill made in accordance with U.S. Pat. No. 6,997,651; and

FIG. 21 is a chart showing a comparison of the edge chippingmeasurements of the tools of FIGS. 18-19.

FIG. 22 is an enlarged perspective view of a portion of a rotary cuttingtool according to one embodiment without a gash blend.

FIG. 23 is a view of a tool similar to FIG. 22 except according to anembodiment with a gash blend.

FIG. 24 is an enlarged perspective view of a portion of a rotary cuttingtool according to another embodiment without a gash blend.

FIG. 25 is a view of a tool similar to FIG. 24 except according to anembodiment with a gash blend.

FIG. 26 is a schematical representation of a portion of a rotary cuttingtool according to an embodiment without a gash blend.

FIG. 27 is a schematical representation of a portion of a rotary cuttingtool according to an embodiment with a gash blend.

FIG. 28 is a table, with reference to FIG. 27, exemplifying dimensionsof a gash blend for a variety of rotary cutting tools according toseveral embodiments.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, an embodiment of the rotary cutting tool or endmill 10 of the present invention is shown comprising a generallycylindrical body 20 having a shank 22 and a cutting portion 24. Thecutting portion 24, also representing the length of cut of the end mill,includes a plurality of flutes 30 formed as pairs ofdiametrically-opposed, symmetrical, helical flutes 32, 34 formed in thebody 20. The flutes 30 of the cutting portion 24 are of the variablehelix type such that the pitch, or index angle, between adjacent helicalflutes 30 is variable along the axial length of the flutes 30. In theparticular embodiment shown, a four flute end mill is shown wherein thefirst pair of diametrically-opposed, symmetrical, helical flutes 32 areformed at a constant helix angle θ of thirty-five degrees and the secondpair of diametrically-opposed, symmetrical, helical flutes 34 are formedat a constant helix angle φ of thirty-eight degrees. The invention isnot limited to these particular helix angles nor is it limited toconstant helix angles as shown in this embodiment and variable helixangles helices are also contemplated.

The end mill 10 further comprises a plurality of peripheral cuttingedges 40, the peripheral cutting edges 40 are formed along anintersection of a circumferential surface, or land, of the cylindricalbody 20 and an inner surface of a respective one of the helical flutes30 facing in a direction of rotation of the body 20. Referring to thecross-sectional view of FIG. 2, the first pair of diametrically-opposed,symmetrical, helical flutes 32 have diametrically-opposed, peripheralcutting edges 42 and the second pair of diametrically-opposed,symmetrical, helical flutes 34 have diametrically-opposed, peripheralcutting edges 44. As mentioned above, the pitch or index angles,designated γ, ε vary in the axial direction and in the cross-section areshown as non-ninety degree angles. In the embodiment shown, γ isequivalent to ninety-three degrees and ε is equivalent to eighty-sevendegrees. Although not shown in cross-section, the pitch or helix anglesγ, ε may be equal in a single radial plane along the length of cut 24.In one embodiment, the radial plane of equal index angles is through themidpoint 26 of the length of cut 24. Although two pairs of diametricallyopposed, peripheral cutting edges 42, 44 are shown, it is contemplatedthat more pairs could be used in other end mill embodiments.

Referring to FIG. 3, a cutting end view of the end mill 10 is shown. Thecutting end comprises a plurality of end cutting edges 60 located on anaxial distal end of the body 20 and contiguous with a corresponding oneof the plurality of peripheral cutting edges 40. Like the spacing of theperipheral cutting edges 40, the index angles γ, ε are shown asnon-ninety degree angles. The end cutting edges 60 all have anequivalent axial rake angle.

As best shown in FIG. 4, the peripheral cutting edges 42 of the firstpair of diametrically-opposed, symmetrical, helical flutes 32, each havea radial rake angle α, while the peripheral cutting edges 44 of thesecond pair of diametrically-opposed, symmetrical, helical flutes 34each have a radial rake angle δ. In the embodiment of FIG. 4, radialrake angle α is different from radial rake angle δ, and moreparticularly, radial rake angle α is three degrees and radial rake angleδ is seven degrees.

In the embodiment of FIG. 5, radial rake angle α is neutral while radialrake angle δ is positive, and more particularly, radial rake angle α iszero degrees and radial rake angle δ is seven degrees.

In the embodiment of FIG. 6, radial rake angle α is negative whileradial rake angle δ is positive, and more particularly, radial rakeangle α is negative seven degrees and radial rake angle δ is positiveseven degrees.

In another embodiment represented by the cross-section of FIG. 7, theradial rake is initially formed as a positive rake angle α, then on atleast one peripheral cutting edge is formed with a radial rake angle δis formed as a K-land of width X such that as shown, radial rake angle αis positive eight degrees and radial rake angle δ is positive threedegrees. It is contemplated that any or all of the peripheral cuttingedges 40 can be formed as K-lands.

Referring now to FIG. 8, at least one of the helical flutes 30 is shownhaving radial rake angles that vary in the axial direction of the flute30. Radial rake angle λ is shown toward the leading end of the flute 30,radial rake angle ψ is shown at the midpoint of the flute 30, and radialrake angle ω is shown toward the trailing end of the flute 30. In theembodiment shown, radial rake angle λ is three degrees, radial rakeangle ψ is five degrees, and radial rake angle ω is eight degrees. Thepresent invention is not limited to the embodiment shown and it iscontemplated that any rake angle that varies in the axial direction ofthe flute is contemplated. For example, the radial rake angle in theaxial direction of the flute may vary from negative, through neutral,and back to positive. Another example is that the radial rake angle inthe axial direction of the flute may vary in different negative amounts.

In another embodiment of the invention as best shown in FIG. 9, theradial rake angles are all different. Accordingly, peripheral cuttingedges 42 of the first pair of diametrically-opposed, symmetrical,helical flutes 32, have different radial rake angles α, σ, while theperipheral cutting edges 44 of the second pair of diametrically-opposed,symmetrical, helical flutes 34 have different radial rake angle δ, ρ. Inthe embodiment of FIG. 9, radial rake angle α is three degrees, radialrake angle δ is three degrees, radial rake angle σ is nine degrees, andradial rake angle σ is seven degrees. However, the invention is notlimited to these values as the radial rake angles α, σ, δ, ρ can be anycombination of positive, neutral, and negative rake angles. In avariation of this embodiment, the rake angles of diametrically opposingperipheral cutting edges are different, i.e. radial rake angles α≠σ andδ≠ρ whereas α may equal δ and/or ρ; or σ may equal δ and/or ρ.

Referring now to FIG. 10, a variable pitch end mill 110 is shown. Thehelix angles φ of the end mill are all equivalent. Due to thepositioning of the helix angles φ, the pitch ε, γ of adjacent cuttingedges 42, 44 varies as best shown in FIGS. 2 and 3. However, unlike thevariable helix end mills 10, the pitch ε, γ does not change along theaxial length of the end mill. The radial rake angles discussed abovewith relation to the variable helix end mills are applied to thevariable pitch end mill 110 in the same manner.

FIGS. 11 and 12 show the present invention applied to variable helix endmill 10 wherein the pitch or index angles ε are equivalent in one radialplane along the length of cut. In the embodiment shown, the one radialplane is at the midpoint 26 of the length of cut 24. The radial rakeangles discussed above with relation to the variable helix end mills andvariable pitch end mills 110 are applied to the variable helix end mill10 of FIGS. 11 and 12 in the same manner.

The different radial rake angles of the present α, δ, σ, ρ, or λ, ψ, ωmay be formed on adjacent or opposite peripheral cutting edges.Conversely, the same radial rake angles may be formed on adjacent oropposite peripheral cutting edges.

EXAMPLES

Testing in the form of a sound comparison and a surface finishcomparison were conducted to compare the end mill of the presentinvention with a standard variable helix Z-Carb® end mill and also anend mill made in accordance with U.S. Pat. No. 6,997,651 havingdifferent axial rake angles and different radial rake angles, but withequal index angles and helix angles. The three end mills each were madeof cemented carbide and having four flutes and a tool diameter of 0.5inch. A chart comparison of radial rake angles and helix angles is shownbelow and identified by position on a four flute end mill:

SGS Z-Carb Rake Helix Tooth No. 1 7° 35° Center Cutting Tooth No. 2 7°38° Non-Center Cutting Tooth No. 3 7° 35° Center Cutting Tooth No. 4 7°38° Non-Center Cutting

present invention Rake Helix Tooth No. 1 3° 35° Center Cutting Tooth No.2 8° 38° Non-Center Cutting Tooth No. 3 3° 35° Center Cutting Tooth No.4 8° 38° Non-Center Cutting

6,997,651 Rake Helix Tooth No. 1 6° 40° Center Cutting Tooth No. 2 15°40° Non-Center Cutting Tooth No. 3 6° 40° Center Cutting Tooth No. 4 15°40° Non-Center Cutting

For the initial sound/surface finish comparison, the end mills were usedto cut a 0.5 inch deep slot in 4140 steel having a hardness of 28 HRc ata rotational speed of 2675 rpm and a feed rate of 18 inches per minute.The results for each tool are shown in FIGS. 13-15 and a comparisongraph is shown in FIG. 16. The results show that the amplitude of noisecreated by the U.S. Pat. No. 6,997,651 having different axial rakeangles and different radial rake angles, but with equal index angles andhelix angles is over 18 times that of the end mill of the presentinvention. The results show that the amplitude of noise created by thestandard variable helix Z-Carb® end mill is over 4 times that of the endmill of the present invention. The noise generated during a cut is oftenindicative of the quality of surface finish that will be achieved by thecut. Referring now to FIG. 17, the surface finish measurements arecompared on a graph. The results show that the surface finish of endmill of U.S. Pat. No. 6,997,651 have a surface finish that was 5.5 timesrougher than the surface finish provided by the end mill of the presentinvention. The prior art Z-Carb® end mill at a service finish that was34% rougher than the end mill of the present invention.

An additional surface finish comparison of the tools was conductedwherein, the end mills were used to cut a double pocket in a 4″×4″×10″block of 4140 steel having a hardness of 28 HRc. Pictures of themachined surfaces for each tool are shown in FIGS. 18-20. The doublepocket machining showed an even larger difference than the straightslotting operation. The surface finish of the end mill of U.S. Pat. No.6,997,651 produced a surface finish of 278 Ra that was over 23 timesrougher than the surface finish of 11.7 Ra provided by the end mill ofthe present invention. The prior art Z-Carb® end mill produced a surfacefinish of 109 Ra that was over 9 times rougher than the end mill of thepresent invention.

Another advantage of the end mill of the present invention over the endmill of U.S. Pat. No. 6,997,651 with regard to edge chipping is shown inthe graph of FIG. 21. The end mills were used to cut a 0.5 inch deepslot in 4140 steel having a hardness of 28 HRc at a rotational speed of2675 rpm for 700 inches total at a feed rate of 25 inches per minute.The results show that the edge chipping of the end mill of U.S. Pat. No.6,997,651 was over nine times greater than the edge chipping of the endmill of the present invention.

In conclusion, the prior art end mill of U.S. Pat. No. 6,997,651has aplurality of flutes all having the same helix angle and being equallyspaced about the circumference of the tool (same index angle), buthaving at least two different radial rake angles and at least twodifferent axial rake angles. The prior art Z-Carb® end mill having aplurality of paired helical flutes forming an even number of helicalperipheral cutting edges equally spaced circumferentially in one planewherein the peripheral cutting edges are formed as a plurality of pairsof diametrically opposite cutting edges having the same helix angle andthereby being symmetrical with respect to the axis of the body. Theseprior art end mills are believed to be the two closest prior artreferences. In a simplistic sense, the present invention is acombination of selected features of the prior art end mill of U.S. Pat.No. 6,997,651 and the prior art Z-Carb® end mill in that embodiments ofthe present invention include an end mill combining diametricallyopposed pairs of radial rake angle and diametrically opposed pairs ofunequal helix angles.

The test results obtained with the prior art end mill of U.S. Pat. No.6,997,651 are poor when compared to the prior art Z-Carb® end mill. Theprior art end mill testing would seem to suggest that changing theradial rake angle of two diametrically opposite pairs of rake angles (asin prior art end mill of U.S. Pat. No. 6,997,651) would not provide anybenefit if combined with diametrically opposed pairs of unequal helixangles (as in the prior art Z-Carb® end mill) and indeed would likelyresult in a decrease in performance.

The test data presented herein shows that the end mill of the presentinvention provides a significant improvement over the prior art andmills, and specifically the Z-Carb® end mill and the end mill of U.S.Pat. No. 6,997,651. The results of the testing using the end mill of thepresent invention are certainly unexpected when looking at theindividual test results of the Z-Carb® end mill and the end mill of U.S.Pat. No. 6,997,651. It is also noted that improvement in end millperformance are typically measured in percent improvement and that a 20to 25% improvement is a significant gain, whereas the improvement in thetest results of the present invention herein are much larger.

There is shown in FIGS. 22 and 23 portions of rotary cutting toolsaccording to different embodiments without a gash blend. Similar tools,except according to embodiments with a gash blends, are shown in FIGS.24 and 25 respectively.

There is shown in FIG. 26 a schematical representation of a portion of arotary cutting tool according to an embodiment without a gash blend andthere is shown in FIG. 27 a schematical representation of a portion of arotary cutting tool according to an embodiment with a gash blend.

There is illustrated in FIG. 28 a table, with reference to FIG. 27,exemplifying dimensions, in inches, of a gash blend for a variety ofrotary cutting tools according to several embodiments. With regards tothe measurements in FIG. 28 and the diagram in FIG. 27, the gash blendis disposed between a corner radius and blend radius, i.e. B-RAD, andextends generally from an axial tangent point C toward a radial tangentpoint A. A width of the gash blend is referenced at 45 degrees betweenthe axial tangent point C and the radial tangent point A. It must beunderstood that the examples shown in FIG. 28 are made with reference toa particular combinations of cutting diameter and corner radii andmerely illustrate a variety of examples within a variety of differentembodiments.

In one embodiment, a tool may have a gash blend with a width referencedat 45 degrees between an axial tangent point and a radial tangent pointthat is between 0.5% and 15.0% of the cutting diameter of the tool. Inanother embodiment, a tool may have a gash blend with a depth orthickness referenced at 45 degrees between an axial tangent point and aradial tangent point that is between 10% and 50% of the corner radius.

Although the present invention has been described above in detail, thesame is by way of illustration and example only and is not to be takenas a limitation on the present invention. Accordingly, the scope andcontent of the present invention are to be defined only by the terms ofthe appended claims.

What is claimed is:
 1. A rotary cutting tool comprising: a plurality ofhelical cutting edges having a common axis and common chirality, a firstpair of helical cutting edges in which each helical cutting edge, isopposed across the common axis by the paired helical cutting edge, has ahelix angle which varies by a first function of axial position, has aradial rake angle that is a function of axial position; and a secondpair of helical cutting edges in which each helical cutting edge, isopposed across the common axis by the paired helical cutting edge, has ahelix angle which varies by a second function of axial positiondifferent from said first function, has a radial rake angle that is afunction of axial position.
 2. The rotary cutting tool of claim 1,wherein said tool has a cutting portion terminating in a cutting end. 3.The rotary cutting tool of claim 2, wherein said helix angle whichvaries by a first function of axial position increases with proximity tosaid cutting end.
 4. The rotary cutting tool of claim 2, wherein saidhelix angle which varies by a first function of axial position decreaseswith proximity to said cutting end.
 5. The rotary cutting tool of claim1, wherein at least one helical cutting edge has a radial rake angle ata first axial position different from the radial rake angle of anotherhelical cutting edge at the first axial position.
 6. The rotary cuttingtool of claim 1, wherein at least one helical cutting edge has a radialrake angle that is a function of axial position that is constant.
 7. Therotary cutting tool of claim 2, wherein at least one helical cuttingedge has a radial rake angle that is a function of axial position thatis variable with axial position.
 8. The rotary cutting tool of claim 7,wherein at least one helical cutting edge has a radial rake angle thatincreases with proximity to said cutting end.
 9. The rotary cutting toolof claim 7, wherein at least one helical cutting edge has a radial rakeangle that decreases with proximity to said cutting end.
 10. The rotarycutting tool of claim 7, wherein at least one helical cutting edge has aradial rake angle at a first axial position different from the radialrake angle of another helical cutting edge at the first axial position.11. A method of forming a rotary cutting tool comprising: providing aplurality of helical cutting edges having a common axis and commonchirality, a first pair of helical cutting edges in which each helicalcutting edge, is opposed across the common axis by the paired helicalcutting edge, has a helix angle which varies by a first function ofaxial position, has a radial rake angle that is a function of axialposition; and a second pair of helical cutting edges in which eachhelical cutting edge, is opposed across the common axis by the pairedhelical cutting edge, has a helix angle which varies by a secondfunction of axial position different from said first function, has aradial rake angle that is a function of axial position.
 12. The methodof forming a rotary cutting tool of claim 11, wherein said tool has acutting portion terminating in a cutting end.
 13. The method of forminga rotary cutting tool of claim 12, wherein said helix angle which variesby a first function of axial position increases with proximity to saidcutting end.
 14. The method of forming a rotary cutting tool of claim12, wherein said helix angle which varies by a first function of axialposition decreases with proximity to said cutting end.
 15. The method offorming a rotary cutting tool of claim 11, wherein at least one helicalcutting edge has a radial rake angle different from the radial rakeangle of another helical cutting edge.
 16. The method of forming arotary cutting tool of claim 11, wherein at least one helical cuttingedge has a radial rake angle that is a function of axial position thatis constant.
 17. The method of forming a rotary cutting tool of claim12, wherein at least one helical cutting edge has a radial rake anglethat is a function of axial position that is variable with axialposition.
 18. The method of forming a rotary cutting tool of claim 17,wherein at least one helical cutting edge has a radial rake angle thatincreases with proximity to said cutting end.
 19. The method of forminga rotary cutting tool of claim 17, wherein at least one helical cuttingedge has a radial rake angle that decreases with proximity to saidcutting end.
 20. The method of forming a rotary cutting tool of claim17, wherein at least one helical cutting edge has a radial rake angle ata first axial position different from the radial rake angle of anotherhelical cutting edge at the first axial position.